Design And Implementation Of An Automatic Method To Extract The Center Flowline Of Glacier

Glacial center flowline,the centerline close to glacial main streamline and used to represent the actual main streamline in the glacier,is the longest flow trajectory of the glacier ice formed in the upper accumulation area moving towards the ice tongue along the bottom of the ice bed,which is one of the important parameters determining the geometry of the glacier.As an important model parameter,the glacial center flowline can be used to determine the change of glacier length with time,analyze the velocity field,estimate glacier volume,and build the one-dimensional model of glacier.Glacier length obtained from it is an important parameter of glacier inventory,which can be used to build ice reserve estimation model with glacier area,and the variation of glacier terminal can be used to analyze the change state of glacier.In this study,founded on the principle of Euclidean allocation theory and the terrain characteristics of glacier surface,a new automatic extraction method of glacial center flowline was designed from the morphological point of view using the glacier outline and digital elevation data.The extraction process mainly includes nine steps:pre-processing of glacier data,extraction of auxiliary lines,determination of outline extreme points,screening of extremely high points,segmentation glacier outline,generation of glacial center flowline,calculation of glacier length and attribute addition.A set of rules was set up in the calculation process,that is,parameters were set for different input glaciers according to the area and perimeter of the glacier outline.All glaciers were logically classified as simple glaciers,simple compound glaciers or complex glaciers and,the process correspondingly ranges from simple to complex.The extraction algorithm involves morphological graph,image feature extraction,image statistics,a tree structure tracing,object the nearest neighbor,iterative search,connection determination,class critical path,etc.The difficulty of the algorithm depends on the complexity of glacier geometry.In order to make the extracted center flowlines more reasonable,this study introduces an auxiliary reference line that expresses the inner ridges of glacier to intervene in the flow line trend of the upper part of the glacier.In addition,glacial center flowline extraction also follows the setting of not passing through the bare rock.Based on the designed algorithm,the involuntary extraction of glacial center flowline was realized using Python tools.The average extraction time of a single glacier is about 20 s.Then,48571 glacial center flowlines in the second glacier inventory of China were extracted automatically by the designed algorithm,and the corresponding the longest length,average length,accumulation area length,and graph data and attribute data of ablation area length were successfully obtained.Through the establishment of an index evaluation system and visual judgment after random sampling,the comprehensive extraction accuracy is 94.34%.Finally,by comparing the longest length(L_max field)of 38053 glaciers matched with the figure position in the world glacier inventory data?RGI v6.0?,from the numerical statistics,the longest length of glaciers extracted in this study is mostly similar to L_max in RGI v6.0,but slightly greater than L_max as a whole.The results showed that:?1?The extraction method designed in this study applies to all single outlet glaciers.The parameters are set according to the classification of glacier morphological characteristics and not intervene in the whole process,realizing the standardization and automatic extraction of glacial streamline.?2?Compared with the longest length of glaciers in RGI v6.0,the glacial longest length obtained by the algorithm of automated extraction of glacial center flowline in this paper is better consistent with the actual situation of glaciers.This is mainly reflected in some glaciers with substantial differences in the longest length,such as the Tugebeliqi glacier.?3?The extraction results of glacial center flowline in some glaciers were inaccurate or wrong,accounting for about 8.28%and 5.66%.Most of these glaciers are data whose contour lines cross the ridge and are not cut into a single glacier,thus,the method of extracting glacial center flowline in this paper can also be used to test the rationality of glacier contour.?4?The length of glaciers in China is mainly less than 5 km,with 46949glaciers,accounting for 96.66%of the total number of glaciers.The glaciers with a length of 0.5-1 km have the largest number,15528 glaciers,accounting for 31.97%of the total.The area and reserves account for 6.21%and 1.79%respectively.The glacier with a length of 2-5 km has the largest area,14753.66 km² in total.The number and reserves of glaciers account for 13.27%and 1.26%respectively.Ice reserves of glaciers with a length of 5-10 km are the most,with a total of 1011.67km³.The number and area of glaciers account for 2.71%and 22.52%respectively.